I notice it differs a lot on what kind of product you’re selling.
The WordPress version of Soon, not so much, maybe 5%. Mostly I guess because it’s a simple installation > Ready!
Improve your documentation and FAQ based on questions you receive, plus add common answers to README are nice ways to lower the amount of support mails you get.
Do keep in mind that support is also I nice way to get into contact with your customers and get to know about the features they really need which you can then use to improve your product further.
I’d say I spent around 3 to 4 hours spent each week on support.